a. Field of the Invention
This invention relates to a system and method for navigating a medical device within a body. In particular, the instant invention relates to a system and method that enable correction of drift and shift in impedance levels in electric field based position and navigation systems.
b. Background Art
A wide variety of medical devices are inserted into the body to diagnose and treat various medical conditions. Catheters, for example, are used to perform a variety of tasks within human bodies and other bodies including the delivery of medicine and fluids, the removal of bodily fluids and the transport of surgical tools and instruments. In the diagnosis and treatment of atrial fibrillation, for example, catheters may be used to deliver electrodes to the heart for electrophysiological mapping of the surface of the heart and to deliver ablative energy to the surface among other tasks. Catheters are typically routed to a region of interest through the body's vascular system. In a conventional approach, an introducer is used to puncture the skin surface and a sheath having an inner diameter greater than the outer diameter of the catheter is threaded through the vasculature to a region of interest. The catheter is then moved longitudinally through the sheath to the region of interest either manually by a clinician or through the use of electromechanical drive systems.
It is desirable to track the position of medical devices such as catheters as they are moved within the body so that, for example, drugs and other forms of treatment are administered at the proper location and medical procedures can be completed more efficiently and safely. One conventional means to track the position of medical devices within the body is fluoroscopic imaging. Fluoroscopy is disadvantageous, however, because it subjects the patient and physician to undesirable levels of electromagnetic radiation. As a result, medical device navigation systems have been developed to track the position of medical devices within the body. These systems typically rely on the generation of electrical or magnetic fields and the detection of induced voltages and currents on position sensors attached to the medical device and/or external to the body. The information derived from these systems is then provided to a physician through, for example, a visual display.
One conventional medical device navigation system is made available under the trademark “ENSITE NAVX” by St. Jude Medical, Inc. The system is based on the principle that when electrical currents are passed through the thorax a voltage drop occurs across internal organs such as the heart and this voltage drop can be measured and used to determine the position of a medical device within the body. The system includes three pairs of patch electrodes that are placed on opposed surfaces of the body (e.g., chest and back, left and right sides of the thorax, and neck and leg) and form generally orthogonal x, y, and z axes as well as a reference electrode that is typically placed near the stomach and provides a reference value and acts as the origin of the coordinate system for the navigation system. Sinusoidal currents are driven through each pair of patch electrodes and voltage measurements for one or more electrodes associated with the medical device are obtained. The measured voltages are proportional to the distance of the device electrodes from the patch electrodes. The measured voltages are compared to the potential at the reference electrode and a position of the device electrodes within the coordinate system of the navigation system is determined.
The above-described system can be used to provide a substantially accurate indication of the position of the medical device within a body. Electric field based navigation systems, however, are subject to various types of interference that can impact the accuracy of position measurements. For example, the level of electrical impedance in the patient body is not necessarily constant. The impedance can slowly drift or even undergo transient shifts due to, for example, a change in medication leading to drift and/or shift in the detected position of the medical device. Various methods have been proposed to mitigate potential drift or shift including bio-impedance scaling, patch center subtraction and the use of a fixed reference catheter with a reference electrode. Bio-impedance scaling and patch center subtraction help to reduce drift and shift, but do not eliminate all cases of drift and shift. The use of a fixed reference catheter requires insertion of an additional catheter into the body thereby increasing procedure time and the risk of complications. Further, the reference catheter may become dislodged during the procedure.
There is thus an ongoing a need for a system and method for navigating a medical device within a body that will minimize and/or eliminate one or more of the above-identified deficiencies.